基于多年观测研究，南海CO2源汇及其时空格局的总体特征是：南海海盆是大气CO2的弱源区，年均海-气CO2通量为2.1±0.3 mmol·m-2·d-1；而南海北部陆架是碳汇区，年均CO2通量为-2.2±3.5 mmol·m-2·d-1；南海总体上每年向大气释放的碳量为1330万±1880万t。由于南海位于陆地-大洋交界带，存在多个界面过程，根据物质交换发生的不同界面，可将南海海盆和北部陆架视为大洋主控型边缘海（OceMar）和河流主控型陆架海（RiOMar）。这两类系统分别接受大洋和河流输入的外源无机碳和营养盐，经由一系列动力过程进入真光层后同时被生物消耗，无机碳和营养盐之间的“竞争”最终决定CO2源汇格局。在南海海盆，无机碳相对过剩，部分以CO2形式向大气释放，即为源；而在南海北部陆架，无机碳相对不足，系统需从大气补充CO2，即为汇。南海碳循环机理及其框架对于更好地理解全球其他陆架边缘海系统具有重要的借鉴意义。
This paper reviews the spatially and temporally varied air-sea CO2 fluxes in the South China Sea (SCS), to show that its basin area is a weak source to the atmospheric CO2, while its northern shelf is a CO2 sink. On an annual average basis, the SCS emits carbon of (1.33±1.88)×1010 g. The northern shelf includes a River-dominated Ocean Margin (RiOMar) during the peak discharges, and an SCS basin as an Ocean-dominated Margin (OceMar). The OceMar is characterized by dynamic exchange with the open ocean via a two-dimensional or even three-dimensional process, i.e., the horizontal intrusion of the open ocean water and the subsequent vertical mixing and upwelling. Depending on the different ratios of the dissolved inorganic carbon (DIC) and nutrients from the source waters into the margins, the relative consumption or removal between the DIC and the nutrients, while being transported into the euphotic zones taken over by biogeochemical processes, determines the CO2 fluxes. Thus, the excess DIC relative to the nutrients in the upper layer will lead to the CO2 degassing. Similar diagnosis can also be made to the RiOMar systems with typical features of significant excess nutrients relative to the DIC. It is suggested that the framework of the carbon cycle revealed from the SCS has important implications in better understanding world's other coastal systems.
[1] Liu Z Q,Gan J P. Three-dimensional pathways of water masses in the South China Sea:A modeling study[J]. Journal of Geophysical Research-Oceans, 2017, 122(7):6039-6054.
[2] Wong G T F, Ku T L, Mulholland M, et al. The South East Asian Time series study (SEATS) and the biogeochemistry of the South China Sea:An overview[J]. Deep Sea Research-Part II, 2007, 54(14):1434-1447.
[3] Li Q, Guo X H, Zhai W D, et al. Partial pressure of CO2 and air-sea CO 2 fluxes in the South China Sea:Synthesis of an 18-year dataset[J]. Progress in Oceanography, 2020, 182, doi.org/10.1016/j.pocean.2020.102272.
[4] Zhai W D, Dai M H, Chen B S, et al. Seasonal variations of air-sea CO 2 fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements[J]. Biogeosciences, 2013, 10:7775-7791.
[5] Du C J, Gan J P, Hui R C, et al. Dynamics of dissolved inorganic carbon in the South China Sea:A modeling study[J]. Progress in Oceanography, 2020, 186:102367.
[6] Dai M H, Cao Z M, Guo X H, et al. Why are some marginal seas sources of atmospheric CO2[J]. Geophysical Research Letter, 2013, 40(10):2154-2158.
[7] Redfield A C, Ketchum B H,Richard F A. The influence of organisms on the composition of sea-water[J]. The Sea, 1963, 2:26-77.
[8] Chen Y L. Spatial and seasonal variations of nitratebased new production and primary production in the South China Sea[J]. Deep Sea Research I, 2005, 52(2):319-340.
[9] Han A Q, Dai M H, Gan J P, et al. Inter-shelf nutrient transport from the East China Sea as a major nutrient source supporting winter primary production on the northeast South China Sea shelf[J]. Biogeosciences, 2013, 10(12):8159-8170.
[10] Wang G Z, Dai M H, Shen S S P, et al. Quantifying uncertainty sources in the gridded data of sea surface CO2 partial pressure[J]. Journal of Geophysical Research:Oceans, 2014, 119(8), doi:10.1002/2013JC009577.
[11] Cao Z M, Yang W, Zhao Y Y, et al. Diagnosis of CO2 dynamics and fluxes in global coastal oceans[J]. National Science Review, 2020, 7(4):786-797.
[12] Cao Z M, Dai M H, Evans W, et al. Diagnosing CO2 fluxes in the upwelling system off the Oregon-California coast[J]. Biogeosciences, 2014, 11(22):6341-6354.
[13] Chou W C, Tishchenko P, Chuang K Y, et al. The contrasting behaviors of CO2 systems in river-dominated and ocean-dominated continental shelves:A case study in the East China Sea and the Peter the Great Bay of the Japan/East Sea in summer 2014[J]. Marine Chemistry, 2017, 195, doi:10.1016/j.marchem.2017.04.005.
